1. Field of the Invention
The present invention relates to a process for producing a propylene block copolymer. More particularly, the present invention relates to a process for producing a propylene block copolymer having a high rigidity and an excellent impact strength.
2. Description of the Related Art
A two step process for producing a propylene block copolymer is known. In this two step process, first, a stereoregular propylene polymer or copolymer is prepared by homopolymerizing propylene or by copolymerizing propylene and another .alpha.-olefin monomer; and second, propylene and another .alpha.-olefin are copolymerized in a gas phase in the presence of the propylene homopolymer or copolymer prepared in the first step.
The product produced by the above-mentioned conventional process consists of a uniform and tight mixture of the polymers and copolymers prepared in the first and second steps. However, this polymeric mixture is commonly referred to as a propylene block copolymer.
This conventional process is useful for producing a propylene block copolymer having a well-balanced rigidity and impact resistance.
However, since this conventional process is composed of a first step of polymerization of propylene in a liquid phase and a second step of copolymerization of propylene and another .alpha.-olefin monomer in a gas phase, it is most important to maintain the catalytic activity of the catalyst in the second step at a high level. Each of the first and second steps may be carried out in a plurality of polymerization reactors.
Further, the conventional process is disadvantageous in that in the second step, since the stickness of the polymerization mixture increases and the fluidity of the mixture reduces, the reactor and related conduit lines are sometimes blocked and scale is formed on the inside surfaces of the reactor and conduits, and since the monomer concentration is low, the productivity per unit time is very low. Accordingly, it is strongly demanded to solve the above-mentioned problems.
In the stereoregular polymerization of propylene, generally, a supported type polymerization catalyst having a high polymerization catalytic activity at an initial style in the use of the catalyst is used. However, this catalyst has a poor retention in the catalytic activity. Accordingly, in the second step of the propylene block copolymer-producing process, the catalyst exhibits a reduced catalytic activity, and thus the production of the propylene block copolymer at a high yield is difficult.
To increase the yield of the copolymer in the second step, the following attempts have been made.
(1) The polymerization time in the first step is shortened to restrict the deactivation of the catalyst during the first step and thus to increase the yield of the copolymer in the second step.
In this attempt, however, the total catalytic efficiency of the polymerization catalyst reduces and the amount of catalyst residue remaining in the resultant copolymer increases. Therefore, the resultant product has an unsatisfactory quality.
(2) The polymerization temperature in the first step is lowered to restrict the deactivation of the polymerization catalyst and thereby to increase the yield of the copolymer in the second step.
In this attempt, however, since the total catalytic efficiency of the catalyst reduces and the stereoregularity of the resultant copolymer decreases, the resultant block copolymer exhibits a reduced rigidity and contains an increased amount of the catalyst residue.
(3) The copolymerization temperature in the second step is raised to enhance the yield of the copolymer in the second step.
In this attempt, the yield of the copolymer in the second step can be enhanced. However, since the elastomeric component in the resultant copolymer exhibits a low molecular weight, an enhancing effect in the impact strength is unsatisfactory and the resultant copolymer exhibits an undesirably increased stickiness.
(4) The second step is carried out under a raised copolymerization pressure to increase the yield of the copolymer in the second step.
In this attempt, however, the polymerization procedure in the second step must be carried out in a high pressure-resistant apparatus which is extremely expensive and thus the cost of the resultant block copolymer becomes high.
(5) The copolymerization time of the second step is made longer.
In this attempt, since the polymerization rate in the second step is lower than that in the first step, the long polymerization time in the second step undesirably causes the total productivity of the process to be reduced.
(6) With respect to a mixing ratio of ethylene to propylene in the second step, ethylene is used in a larger amount than that of propylene.
In this attempt, the proportion in amount of a random copolymer to the whole copolymer produced in the second step is reduced, and the content of ethylene homopolymer contained in the copolymerization product of the second step increases. Therefore, the enhancing effect in the impact strength of the resultant propylene block copolymer is undesirably reduced.
(7) During the copolymerization procedure in the second step, a specific compound is added to the copolymerization mixture. (Japanese Unexamined Patent Publication (Kokai) No. 53-30686)
In this attempt, when a specific compound, for example, an organic aluminum compound is added, the resultant polymer particles in the reactor exhibit an increased stickiness and thus are adhered to each other to form lumps and the operation of the reactor becomes difficult to continuously carry out.
Also, it was attempted to add an aluminum alkoxide to the polymerization mixture. This attempt is, however, disadvantageous in that the stereoregularity of the resultant copolymer is significantly reduced and the resultant copolymer particles are easily adhered to each other.
To prevent the adhesion of the copolymer particles to each other, Japanese Unexamined Patent Publication (Kokai) Nos. 62-132,912, 62-135,509 and 2-117,905 disclose the use of an alkyl lithium or alkyl magnesium. This attempt is, however, disadvantageous in that the use of the above-mentioned compounds causes the resultant copolymerization mixture to exhibit a significantly reduced catalytic activity.
To prevent the adhesion of the copolymer particles to each other and enhance the copolymerization activity of the copolymerization mixture, Japanese Unexamined Patent Publication (Kokai) No. 58-213,012 discloses the co-use of an aluminum alkoxide and a hydrocarbon, and clearly stresses that if the abovementioned compounds are not used altogether, are desired effects do not appear.
To improve the fluidity of the block copolymer, Japanese Unexamined Patent Publication (Kokai) No. 61-69,823 discloses an addition of a metal alkoxide, Japanese Unexamined Patent Publication (Kokai) No. 61-215,613 discloses an addition of a compound having a Si--O--C bond, Japanese Unexamined Patent Publication (Kokai) No. 63-146,914 discloses an addition of a siloxane compound, and Japanese Unexamined Patent Publication (Kokai) Nos. 3-292,311 and 4-136,010 disclose an addition of a silicon compound. These attempts are disadvantageous either in reduction of activity of the resultant polymerization mixture or in reduction of stereoregularity of the resultant copolymer.
Also, to restrict a production of by-products and improve a processability of the resultant block copolymer, an addition of a specific electron donor is attempted in Japanese Unexamined Patent Publication (Kokai) Nos. 56-151,713, 61-69,821, 61-69,822, 61-69,823, and 63-43,915.
Also, Japanese Unexamined Patent Publication (Kokai) No. 4-331,219 discloses an attempt to feed a saturated hydrocarbon having 3 to 5 carbon atoms into the copolymerization mixture.
However, these attempts are unsatisfactory in that the polymerization activity of the polymerization mixture and/or the molecular weight of the resultant copolymer is reduced, and the process cost becomes high because the additive must be employed in a large amount.